The present invention relates to a method and an apparatus for synchronous demodulation of multiply modulated signals.
An electromechanical rotation rate sensor, for example a micromechanical rotation rate sensor, constitutes a spring-mass system in which the Coriolis effect may be used to measure the rotation rate of the sensor. Upon rotation of the sensor or the system, the masses of the system are deflected and the deflection may be ascertained capacitatively in order to determine the rotation rate. While the rotation rate sensor is in operation, the spring-mass system of the rotation rate sensor oscillates at its natural resonant frequency fz. Absent the effect of an external rotation rate, the capacitance of so-called sensing capacitances provided in the rotation rate sensor does not change. If the spring-mass system is deflected xe2x80x9cout-of-planexe2x80x9d in the context of a rotation of the sensor, the capacitance changes at the natural resonant frequency fz. The amplitude of this oscillation is an indication of the external rotation rate presently acting on the rotation rate sensor.
The deflection of the spring-mass system during its rotation as a result of the Coriolis effect may be sensed, by way of a capacitance change associated therewith in capacitances provided in the rotation rate sensor, by way of a capacitance/voltage (C/U) conversion.
When capacitance changes occur as a result of the rotation of the sensor, those changes may bring about voltage jumps at the input of the charge amplifier. The output signal Uout of the charge amplifier is proportional to the quotient of the useful capacitance CNUTZ and feedback capacitance CRK of the sensor in question, multiplied by the amplitude of the high-frequency voltage jump UHF, that is:
UOUT=(CNUTZ/CRK)*UHF. 
To ensure that signal processing of the output signal UOUT of the charge amplifier can occur in phase with the natural resonant frequency fz of the spring-mass system, a phase-locked loop (PLL) that synchronizes itself to the natural resonant frequency fz of the spring-mass system may be used. The sampling frequency for analyzing capacitance changes by the switched capacitor technology may be referred to as fa, and the modulation frequency, or frequency of the high-frequency voltage jump UHF, may be referred to as fHF.
To achieve a greater signal-to-noise ratio than would be possible in the baseband, the voltage jumps that are brought about by the capacitance changes may be alternated as to sign. The baseband is thereby transformed to half the sampling frequency fa, that is, to the frequency fHF of the voltage jump UHF; thus fHF=fa/2. In same analysis devices, the natural resonant frequency fz of the rotation rate sensor may be used as the intermediate frequency. In a first demodulation step in synchronous demodulation, the raw signal is multiplied by the frequency fHF of the voltage jump UHF. In a second demodulation step of synchronous demodulation, the rotation rate signal of the rotation rate sensor is then convoluted into the baseband. In a subsequent filtration step, the high-frequency convolution products are suppressed and the output signal of the filtration step or of a corresponding filter stage is limited to the desired bandwidth.
For cost reasons, according to one exemplary embodiment and/or exemplary method of the present invention, a charge amplifier of so-called switched capacitor technology may be used as the C/U converter.
According to an exemplary embodiment and/or exemplary method of the present invention the high-frequency signals of frequency fHF, which may serve for analysis of the capacitance change resulting from the Coriolis effect upon rotation of the rotation rate sensor, may be configured by way of a PLL as a multiple of the signal, of natural resonant frequency fz, of the spring-mass system of the rotation rate sensor. A fixed dependence between the phase position of the signals and frequencies fHF and fz may thereby obtained. As a result of a logical association between these two signals, signals for controlling a synchronous demodulator may be generated. This may be done with the exemplary circuit according to the present invention depicted in FIG. 1. This circuit may create an in-phase multiplication of the two frequencies fHF and fz and may effect a synchronous demodulation that transforms the rotation rate signal to be sensed, or the rotation rate signal of the rotation rate sensor, into the baseband.
An exemplary embodiment and/or exemplary method of the present invention is directed to providing a method for synchronous demodulation of the multiply modulated rotation rate signal of a rotation rate sensor which includes a spring-mass system that oscillates at its natural resonant frequency (fz), and at least one capacitor for ascertaining a Coriolis effect acting on the spring-mass system; i.e. a rotation rate, the rotation rate being ascertained by a time-variable capacitance change, brought about by the rotation rate, of the at least one capacitor by multiple demodulation of a multiply modulated electrical signal that includes a time-variable first electrical signal (fHF) and a second electrical signal, superimposed thereon, at the natural resonant frequency (fz) of the rotation rate sensor, the amplitude of the second electrical signal of the time-variable capacitance change of the at least one capacitor being correspondingly modulated, wherein the first electrical signal (fHF) is generated in such a way that it exhibits a time-invariant phase relationship to the natural resonant frequency (fz) of the rotation rate sensor.
The following components may be used to provide an exemplary embodiment and/or exemplary method according to the present invention: a rotation rate sensor, a PLL, a logic circuit, and a synchronous demodulator, as depicted, for example, in FIG. 1. Several demodulation sections with mixers, filters, and optionally also intermediate amplifiers may be necessary.
According to one exemplary embodiment and/or exemplary method of the present invention, the outlay for synchronous demodulation may be almost halved.